It is well known that microcircuit fabrication requires the formation of precisely controlled openings, such as contact openings or vias, which are subsequently interconnected to create components and very large scale integration (VLSI) or ultra large scale integration (ULSI) circuits. Equally well known is that the patterns defining such openings are typically created by optical lithographic processes, which involve the use of a mask and radiation, such as ultraviolet light, electrons or x-rays, to expose a pattern in the photoresist material. The exposed patterns in the photoresist are formed-when the wafer undergoes the subsequent development step. The exposed portion of the photoresist is removed and unexposed portions of the photoresist remains to protect the substrate regions that it covers. Locations from which photoresist has been removed can then be subjected to a variety of subsequent processing steps.
Formerly, in technologies involving features of greater than 0.5 microns, the degree of resolution was not as critical and longer wavelengths, such as those around 600 nm could be used. Accordingly, the equipment used in these conventional lithographic processes was developed to accommodate these design parameters.
In today's deep sub-micron technologies, however, the degree of resolution that can be achieved by such lithographic processes factor in consistently printing minimum size images has become even more critical in deep sub-micron circuits with features less than 0.5 .mu.m. Thus, the fabrication of increasingly smaller features on VLSI or ULSI relies on the availability of increasingly higher resolution lithography equipment or processes. This higher resolution may be achieved in several ways. For example, the illuminating wavelength can be decreased, or the numerical aperture of the system lens can be increased. The contrast of the photoresist can also be increased, by modifying the photoresist chemistry, by creating entirely new resists, or by using contrast enhancement layers, which allows a smaller modulation transfer function to produce adequate images. Alternatively, the coherence of the optical system can be adjusted.
As features sizes have become smaller, difficulty in controlling the appropriate amount of photoresist exposure has increased due to stray light problems associated with patterning these smaller features. In some cases, over exposure of the desired photoresist area may occur, and in other cases, under exposure of the photoresist area may occur. In either case, critical dimension (CD) line width control becomes more difficult.
Accordingly, optical lithography for deep sub-micron integrated circuits with feature sizes less than 350 nm (0.35 .mu.m) requires shorter wavelength exposure (365 nm or 248 nm) of the photoresist materials used for defining circuits. Most recently, new stepper lithography equipment has become available that uses shorter wavelengths to allow more precise exposure of a photoresist that is sensitive to the shorter wavelengths. With this new technology, smaller contact opening and via sizes can be obtained. However, this new stepper equipment is expensive and thus substantially increases the overall manufacturing cost of semiconductors.
Therefore, it is highly desirable to achieve the smaller contact openings and vias required by today's deep sub-micron technologies without the additional start-up costs associated with the new stepper technology and yet avoid the problems with inconsistent exposure of the photoresist.